Novel intervention strategies are needed to reduce HIV incidence in the US below the current 40K plateau. The highly-infectious acute phase of initial infection represents a promising but relatively unexplored target for behavioral change interventions. Interventions that identify acutely-infected persons and help them reduce their risk of transmitting the virus to others could play an important role in efforts to reduce HIV incidence below the 40K plateau. However, the ultimate impact of these interventions depends on the actual extent of transmission during the acute phase of infection, as well as on the cost-effectiveness of potential interventions to reduce acute-phase HIV transmission. To enhance current understanding of the epidemiological impact of acute-phase HIV transmission and the cost-effectiveness of potential strategies to reduce HIV transmission during this period of elevated infectiousness, we will: 1) Develop mathematical models and procedures to estimate the annual number of incident HIV infections due to acute-phase transmission, the total number of such infections from 1998 through 2004, and the potential epidemiological and economic impact of interventions to reduce acute-phase HIV transmission. 2) Assess the efficiency and cost-effectiveness of supplementing existing HIV counseling and testing services with HIV-RNA testing of pooled test samples to facilitate identification of acutely-infected persons and encourage behavioral risk reduction to prevent acute-phase HIV transmission. 3) Assess the costs and estimate the number of HIV infections averted by a comprehensive post-exposure prophylaxis program that combined antiretroviral prophylaxis with either a 2-session behavioral risk reduction intervention or a more intensive 5-session intervention. This project will provide estimates of the magnitude and consequences of acute-phase HIV transmission, and the potential economic efficiency of various intervention strategies to prevent transmission during the acute phase of infection. This information is needed by decision makers at federal, state, and local levels who must prioritize HIV prevention intervention development and implementation. The results will inform scientific and policy discussions on the importance of interventions to reduce acute-phase transmission.